1. Field of the Invention
The present invention relates to a process for production of a highly purity silicic acid aqueous solution from a low pure alkali silicate aqueous solution. More particularly, it relates to a process for producing a highly pure silicic acid aqueous solution. Considering the fact that low purity alkali silicates have been thought to be unsuitable as starting materials for silica sources of an artificial quartz or an optical fiber, it is surprising that the present invention can use a low purity silicate for such purposes.
2. Description of the Prior Art
Silica sources for an artificial quartz or an optical fiber have been produced by pyrolysis of purified silicon tetrachloride or by hydrolysis of purified tetraethyl silicate. Though the silicon tetrachloride and tetraethyl silicate have high purities, they are very expensive and difficult to handle because of their corrosiveness and flammability.
A Japanese Patent Laid Open No. Sho. 63-21212 discloses a process for producing silica comprising, ion exchanging an alkali silicate aqueous solution with cation-exchange resin to get an acidic silica sol, adding an acid to the sol, ion exchanging the sol with cation-exchange resin, and pouring the thus treated sol into an ammonium-containing alkaline solution to precipitate silica.
A Japanese Patent Laid Open No. Sho. 60-191016 also discloses a process for producing precipitated silica by adding a strong acid to an alkali silicate to obtain a free acid concentration of 1 N or more, and precipitating silica at a high temperature from 70.degree. C. to 90.degree. C.
Japanese Patents Laid Open No. Sho. 60-2041613 and Japanese Patents Laid Open No. Sho. 60-2041614 also disclose processes which comprise producing silica gel from a concentrated alkali silicate aqueous solution by simply rinsing it, and heating the gel in a strong acid repeatedly.
These prior processes, however, hardly provide a highly pure silica having metal impurity content of 1 ppm or less because they use low purity alkali silicates as starting materials. Consequently, a highly pure (low impurity) alkali silicate aqueous solution must be used as a starting material to produce a highly pure silica.
Non-alkali metal impurities exist as silicate complexes in an alkali silicate solution The complexes are very stable at ordinary temperature; they hardly dissolve in water except when a large amount of hydrochloric acid is added. When a hydrochloric acid is added, non-alkali metals form water soluble metal chlorides. The previously referred Japanese Patent Laid Open No. Sho. 60-191016 uses this reaction. According to this invention, a large amount of hydrochloric acid is added to an alkali silicate solution (silica content, 10% by weight or less) until the solution turns acidic, then the solution is contacted with the ion-exchange resin to remove metal impurities. The metal impurities are removed very efficiently.